1. Field of the Invention
The present invention relates to a switching circuit, a switching module and a method of controlling the switching circuit.
2. Description of the Related Art
FIG. 1 shows a circuit structure of a conventional semiconductor switch 100. As shown in FIG. 1, the conventional semiconductor switch 100 has a structure such that a high-frequency input signal supplied through an input terminal Tin is selectively outputted from one of a plurality of output terminals (Tout1, Tout2). In FIG. 1, the semiconductor switch 100 adopts an SPDT (single pole double throw) arrangement.
The semiconductor switch 100 may be applied to, for example, an arrangement in which an antenna is shared by a transmission circuit and a reception circuit. The switch 100 cuts off electrical connection between the antenna and one of the two circuits while the other circuit is using the antenna.
A source voltage Vs of a non-selected switching transistor (Tr1/Tr2) depends on the potential of the source of a selected switching transistor (Tr2/Tr1) defined by applying a voltage to the gate of the selected transistor. Therefore, a potential difference is produced between the gate and source of the non-selected switching transistor (Tr1/Tr2), and maintains the non-selected switching transistor (Tr1/Tr2) in the cutoff (OFF) state.
When all output terminals (Tout1, Tout2) are cut off in the above-mentioned configuration, it is impossible to set the source voltage Vs at a prescribed level because there are no switching transistors in the selected state. This leads to a problem that the switching transistors cannot be fully cut off.
FIG. 2 shows another conventional switching circuit 200 in which a bias voltage Vba is constantly supplied the sources of the transistors Tr1 and Tr2 via a resistor R5. The bias voltage Vba sets the potential difference between the gate and the source of the switching transistors (Tr1, Tr2) at a prescribed level. Japanese Laid-Open Patent Application No. 2000-223902 discloses a technique for solving the same problem as described above.
However, the above-mentioned conventional techniques are intended to merely keep the source voltage Vs of the non-selected switching transistor at a prescribed level when any of the switching transistors is in the selected state. Therefore, when all the switching transistors are in the non-selected state, these transistors cannot be fully cut off.